Memory polymer optical fiber splicer and methods

ABSTRACT

The present invention involves an optical fiber splicer made of a unistructural mass of inherent shaped memory polymer material. The splicer has a longitudinal dimension with opposite ends having first bore at one end and a second bore at the other end wherein the bores go into the mass and to each other. Preferably, the first bore and the second bore are merely a single continuous oriface. The unistructural mass has a first shape and a second shape. The first shape is a recoverable, predetermined inherent shape wherein the first bore and second bore each have a preset diameter to accomodate and tightly hold end segments of denuded optical fibers of predetermined diameter in spliced, butted alignment with one another. The second shape is such that the first bore and second bore each have swollen predetermined diameters which are greater than the diameters of the end segments of optical fibers so as to loosely and freely receive the end segments. The unistructural mass of inherent shape memory polymer material is initially formed in the first, inherent shape, and is then swollen and partially shrunk to its second, deformed shape and is capable of being returned to its first shape by application of a non-mechanical stimulus thereto, such as heat. The invention is also directed to preparing the optical splicer as well as to using the optical splicer to obtain a butt-to-butt splicing of optical fibers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to splicing of optical fibers and morespecifically to the butt-to-butt splicing of optical fibers by the useof a unique splicer lock which is developed from memory polymers. Thus,the present invention is directed to splicers into which ends of opticalfibers may be inserted and subsequently spliced in place in abutt-to-butt manner for permanent fiber optics transmission and tomethods of making the splicer as well as methods of using it.

2. Prior Art Statement

Hundreds of patents have issued which are directed to the art of fiberoptics splicing and connecting. Most involve very complex mechanicalconnections and/or the use of optical adhesives or glues. Typical of thepatents which show unique methods of splicing are the following:

U.S. Pat. No. 3,944,328 shows the simplistic approach of inline,butt-to-butt splicing of fiber optics utilizing a resinous block withaligned bores with mechanical retention inserts.

U.S. Pat. No. 4,178,067 involves the use of a mass of dimensionallyunstable material in cylindrical form which is radially shrunk. Afterthe fiber optics are inserted butt-to-butt, upon causing the unstablematter to expand radially, the presence of the outer sleeve forces theunstable matter to compress and cause co-linear alignment.

U.S. Pat. No. 4,261,644 involves the use of memory metals formechanically splicing with application of heat.

U.S. Pat. No. 4,435,038 is directed to deformable material involvingthree integrally formed elongated members that are squeezed together toalign optical fibers.

U.S. Pat. No. 4,597,632 is directed to temperature sensitive releasableoptical connector which utilizes a shape memory effect metal to alignand clamp ferrules.

U.S. Pat. No. 4,647,150 illustrates an arched alignment of opticalfibers utilizing an optical adhesive as well as an innertube forbutt-to-butt splicing.

U.S. Pat. No. 4,725,117 describes a complex optical fiber connectionutilizing a heat-recoverable tube which is constructed of a memorymaterial such as elastic or plastic memory materials, as well as anouter metal contact body. The basic idea of shrinking a memory plasticradially inward to achieve optical fiber alignment is taught in thispatent.

U.S. Pat. No. 4,743,084 involves improvement in the use of deformableplastics or the use of shape memory materials as an integral part of amore complex mechanic structure.

U.S. Pat. No. 4,750,803 describes a connector which includes exit portmeans to allow air to escape during splicing.

In addition to the above, there were suggested many splicing methodsusing heat-shrinkable polymers (called also memory polymers,heat-recoverable polymers, etc.) to help align optical fibers. Thesepolymers (described e.g. in U.S. Pat. Nos. 2,027,962; 3,086,242;3,359,193; 3,370,112; 3,597,372 and 3,616,363) are either thermoplastsor post-crosslinked thermoplasts containing a crystalline polymer phaseand/or amorphous polymer phase with relatively low glass-transitiontemperature due to either nature of the polymer or due to plastificationeffect. Such heat-shrinkable polymer can be forced into one shape andfrozen in it; and shrunk by application of heat approximately into theoriginal shape. Because such heat-shrinkable polymers consist of severalpolymer phases and/or a multitude of separate polymer chains, they donot have a precise shape which could be called "inherent" and they canreturn only approximately into a predetermined shape. In addition tothat, the highly crystalline polymers must be heated above the meltingpoint of their crystalline phase; the re-crystallization of the polymercauses significant volume contraction which is detrimental to thealignment. Because of that the heat-shrinkable component in itselfcannot form a splicer with a low insertion loss. Such splicers require acombination of highly symmetric tubular shape, coupling gels oradhesives and elaborate support structures, which in turn cause a highcost of the device and of its installation. Typical of the patents whichshow the unique methods of splicing are the following: Great BritainPatent No. 1,588 227 describes a splicing method using heat-shrinkablecrystalline thermoplastic sleeve to achieve fiber connection.

Notwithstanding the above prior art references, it should be noted thatthe present invention has not been anticipated or rendered obviousbecause the use of a unistructural memory polymer is neither disclosednor suggested as a complete and simple but advanced structure in and ofitself. Further, the unique steps of preparing the memory polymerutilized in the present invention splicer is also lacking in the priorart.

SUMMARY OF THE INVENTION

The present invention involves an optical fiber splice which is made ofa unistructural mass of inherent shape memory polymer material. Theunistructural mass has a longitudinal dimension with opposite ends whichincludes a first bore at one end and a second bore at the other endwherein the bores go into the mass and to each other. In a preferredembodiment, the first bore and the second bore are merely a singlecontinuous oriface. The unistructural mass has a first shape and asecond shape and the first shape is a unique, recoverable, predeterminedinherent shape wherein the first bore and second bore each have a presetdiameter to accommodate and tightly hold end segments of denuded opticalfibers of predetermined diameter in spliced, butted alignment with oneanother. The second shape is a deformed shape caused by solvent swellingand partial shrinking of the inherent memory polymer material such thatthe first bore and second bore each have swollen predetermined diameterswhich are greater than the diameters of the end segments of the denudedoptical fibers so as to loosely and freely receive the end segments ofthe denuded optical fibers. Thus, the unistructural mass of inherentshape memory polymer material is initially formed in the first shape,also known as the inherent shape, and is then swollen and partiallyshrunk to its second or deformed shape and is capable of being returnedto its first shape by application of a non-mechanical stimulus thereto,such as heat. The invention is also directed to the method of preparingthe optical splicer as well as the method of using the optical splicerto obtain a butt-to-butt splicing of end segments of optical fibers.

BRIEF DESCRIPTION OF THE DRAWINGS

The objectives and advantages of the present invention will become moreapparent and will be more fully understood from the following detaileddescription taken in conjunction with the accompanying drawings wherein:

FIG. 1 shows an oblique frontal view of an optical fiber splicer of thepresent invention;

FIG. 2 shows a cut sideview of an optical splicer of the presentinvention having bores with the unistructural mass in its inherentshape;

FIG. 3 shows the side cut view of the optical fiber splicer of FIG. 2which has been swollen and has a mandrel inserted therein;

FIG. 4 shows a side cut view of the optical fiber splicer of FIG. 2after it has been swollen and partially shrunk;

FIG. 5 shows a cut sideview of the optical fiber splicer of FIG. 2wherein denuded end segments of two optical fibers have been insertedinto the bores;

FIG. 6 shows the optical fiber splicer of FIG. 2, after theunistructural mass has been heated so as to return to its inherentshape, thereby aligning and holding the end segments of the opticalfibers; and,

FIG. 7 shows a side cut view of an alternative optical fiber splicer ofthe present invention adapted to receive different size optical fibers.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The butt-to-butt splicing of optical fibers requires high alignmentprecision. The faces must be flat and perpendicular to optimize lighttransmission and there must be high mechanical integrity for fiberfixation. The outer diameter or cladding diameter of the majority ofcurrently used glass optical fibers is standardized at 125 microns whiletheir light guiding core range is between 4 microns for single modefibers up to about 50 to 100 microns for multi-mode fibers. Thus, anymisalignment of the spliced optical fibers, any defect at the faces ofthe fibers or on the surfaces or any gap between the fiber ends willcontribute to the loss of the transmitted light and to a resulting lossof transmitted information. This transmission loss, referred to as"insertion loss" reduces the signal-to-noise level, and is expressed indecibels (dB). It is an object of all splicing techniques to minimizeinsertion losses.

The application of various splicing techniques such as those which aredescribed in conjunction with the prior art patents set forth aboveunder field conditions is very difficult and requires highly skilled andexpensive personnel as well as splicing products which are fairlyexpensive. Thus, the splice as used for multi-mode applications mayexhibit poor performance notwithstanding the effort put into reducinginsertion losses. Further, the use of multi-mode applications in singlemode fiber optic splicing also results in increased insertion losses.For example, a very common splicing technique involves fusing the twoends of the butt-to-butt optical fibers under a specialstereomicroscope. This procedure provides low loss connections but dueto the considerable set up time requires a substantial amount ofexpenditure and is extremely inconvenient for field applications. Moreportable or in-place splicing equipment seems to yield increasedinsertion losses based on their relative economics. In other words, theless expensive the technique, the more likely significant insertionlosses will occur.

It is an objective of the present invention to achieve a splicer whichwould provide low cost, mass produced unistructural products which wouldbe fast and convenient under field conditions and which would besuitable for use by personnel who do not require special technicalskills or the use of special high performance equipment. Further, thepresent invention splicer achieves low loss performance without usingindex-matching gels or adhesives.

As indicated, the major problem with splicing and connecting opticalfibers in a butt-to-butt arrangement involves misalignment. However, theinherent memory polymer materials utilized in the present inventioneliminate the problem due to accurate mutual positioning of the opticalfibers.

Inherent memory polymers are amorphous with moderate crosslinkingdensity. They are substantially deformable above their softeningtemperature and have only one "inherent shape" in which all of thepolymer segments are in their most probable conformation. Thecrosslinking density is such that each continuous piece of the inherentmemory polymer is formed substantially (i.e. with exclusions ofimpurities such as residual solvents, monomers, oligomers, etc.) by onesingle giant macromolecule. Furthermore, each such macromolecule hassubstantially a uniform network density and substantially a singleamorphous polymer phase. We refer to such structure as a "unistructuralmass" to distinguish it from heat-shrinkable, shape-recovering or memorypolymers having more than one polymer molecule and/or more than onepolymer phase which lack the inherent shape memory. As it is obviousfrom review of the prior art, the typical heat-shrinkable polymerhitherto used for splicing are highly crystalline polymers, such aspolyethylene, copolymers of ethylene-vinylacetate and the like. Thesepolymers consist of two polymer phases of substantially differentproperties. For instance, polyethylene used in most of the applicationshas a crystalline phase with a melting point between about +105° to+130° C., and an amorphous phase with a glass-transition temperaturebetween about -100° C. and -70° C. The amorphous crosslinked polymersused in the splicers according to our invention have an additionaladvantage in their high optical clarity. Because our splicers aretypically clear, uniform plastic articles, the loss in transmitted lightis immediately detectable by the light emitted at the fiber endscontact. This can be used, given proper instrumentation, for measuringthe insertion loss directly and in a manner very suitable for fieldinstallations. The shape of such a polymer can be changed above itssoftening temperature into a deformed shape by the action of externalforces but once the external forces are removed the internal forces ofthe polymer cause it to return to its inherent shape. The deformation ofthe inherent memory polymer is fully reversable and this allows completerecovery to its inherent shape. The return from deformed to inherentshape can be deferred by decreasing temperature on the deformedunistructural mass below its softening temperature. Its return toinherent shape can be then induced by some external stimuli such as anapplication of heat or other energy.

In general, the optical fiber splicer of the present invention is madefrom a mixture of monomers so as to create a unistructural mass whichhas a longitudinal dimension with opposite ends which includes a firstbore at one end and a second bore at the other end and these bores enterthe mass towards and in alignment with one another and, in fact, connector intercept one another. It should be noted that the unistructural massis defined as having two bores, although, preferably, it is a singlecontinuous opening from one end of the unistructural mass to the otherand might be characterized as a single bore. Thus, the two bores of thepresent invention are equivalent to and, by definition, in manyinstances are defined as a single bore which passes completely throughthe unistructural mass.

A typical memory splice made from the inherent memory polymer materialin the present invention may have a continuous lumen which iscylindrical and which has a diameter which is equivalent to or slightlyless than the other diameter of a bare optical fiber to be connectedwith another bare optical fiber. Prior to insertion of the fibers, thebores or lumen is deformed so that its inside diameter becomes largerthan the outside diameter of the fibers to be spliced and theunistructural mass is "frozen" in this deformed state. As long as thememory polymers are frozen in their deformed state by exposure totemperatures below their glass tramsmission temperatures, e.g. bysolvent swelling and partial shrinking, they will remain frozen untilthe application of adequate heat to raise the temperature of theunistructural mass above the glass transmission temperature, at whichtime the unistructural mass will return to its inherent shape.Therefore, the optical splicer of the present invention receives denudedsegment ends of optical fibers which are easily inserted into theenlarged bores. Thereafter, the unistructural mass is exposed to astimulus such as heat which causes the recovery of the inherent shapeand the inherent aligning and tightening of the butt-to-butt opticalfiber end segments. The splicer cools down to ambient temperature andforms a rigid glassy polymer which permanently holds the fiber endssafely, firmly and properly aligned.

In order to achieve a well defined inherent shape the polymer or mixtureof polymers utilized has to have at least a minimum cross linkingdensity. The shape of such polymer can be changed into a deformed shapeby the action of external forces and can be returned to the inherentshape by the action of other external forces. It has been noted ingeneral that the deformability of any polymer with inherent memorydecreases with its cross linking density. To achieve a sufficientreversal of deformability the polymer should, in preferred embodiment,have maximum cross linking density. Thus, the inherent memory polymermaterials used in the present invention have at least a minimum crosslinking density and, ideally, have a maximum cross linking density.

Non-crystalline (i.e. amorphous) polymers have two types of behavior intwo temperature ranges. The transition temperature between the tworanges is called Glass-Transition Temperature (T_(g)). If temperature islower than T_(g), the polymer is in glassy state in which itsdeformability is very low and modulus of elasticity is high. Iftemperature is higher than T_(g), then modulus of elasticity is low anddeformability is high. The polymer in this temperature range hasviscoelastic or rubbery behaviour, depending on crosslinking density andtemperature.

The softening temperature T_(s) is used in further description insteadof T_(g) and T_(m) because it is readily measurable, e.g. by ASTMD569-48 or by a similar method, and because it is more closely relatedto the polymer performance with respect to the present invention.

Memory polymers useful in the present invention are those with T_(s)higher than ambient temperature, and preferably higher than about 50° C.The upper limit of T_(s) is restricted only by temperature resistance ofthe memory polymer, optical fiber or other system components and meansof the connector heating during installation. Therefore, there is noinherent upper limit on T_(s), but above practical consideration set thepractical upper limit on T_(s) to be about 200° C., and preferably about150° C.

Most of non-crosslinked polymers are soluble in one or more solvents.Such solvents are different for various polymers and referred to as"thermo-dynamically good solvents" (TGS). Solvent-polymer interactionsare characterized in various ways, e.g. by Chi parameter inFlory-Huggins eqution. TGS have Chi<0.5 for a given polymer. (In otherwords, if Chi<0.5 for a given polymer-solvent pair, the non-crosslinkedpolymer is soluble in this solvent.)

Covalently crosslinked polymers cannot be dissolved without destructionof the network. Instead, they swell to certain equilibrium in thesolvents with Chi<0.5 and become rubbery in the process. The swellingextent can be expressed in various ways, for instance as volume fractionof polymer v₂ or solvent content B_(s) in weight %. The swelling extentdepends both on value of Chi and on crosslinking density. Because Chivalue does not change with crosslinking appreciably and can beestablished independently of swelling (e.g. measured on linear modelpolymers), the swelling of crosslinked polymers in TGSs can be used ascharacteristic of crosslinking density.

Memory polymers useful in the present invention have minimumcrosslinking density corresponding to v₂ =0.05, and preferably v₂ =0.1in the TGS with Chi=0.3-0.4. The maximum cross linking densitycorresponds to v₂ =0.5, and preferably to v₂ =0.667 for the same TGS.

Swelling of the memory polymer has several effects important to thepresent invention:

1. Swelling in TGS to equilibrium decreases T_(s) below ambienttemperature so that swollen memory polymers are rubbery and readilydeformable.

2. Swelling increases isotropically every inherent dimension of thememory polymer. In the swollen state

    LD.sub.s =LD.sub.i *(1/v.sub.2).sup.1/3                    (1)

where LD_(s) and LD_(i) are the swollen linear dimension in undeformedstate and inherent linear dimension, respectively.

As long as the covalent network is isotropic, the above relation governsall linear dimensions, such as length, outside diameter, inside diameterof a cavity in the memory polymer, etc.

The polymers satisfying the essential requirements can have variouschemical composition. Such inherent memory polymers can be crosslinkedacrylates and methacrylates; N-substituted acryl and methacrylamides;crosslinked vinyl polymers, such as polystyrene, polyvinylpyrridine,polyvinylchloride, resins and the like. In other words, because therequirements are of physical nature, the function of the invention isindependent of the specific composition of the memory polymer as long asthe essential physical requirements are met.

Some specific memory polymer compositions are described in Examples setforth below.

The present invention consists in using inherent shape memory polymersof above described characteristics in conjunction with certain design ofthe connector of optical fiber and certain connecting or installationprocedure. The designs and procedures described below assume that theoptical fiber has circular crossection with coaxial core and jacket,because all current optical fibers are of such kind. It is not intended,however, to limit the invention to connecting the current optical fibersonly. Should another type of fiber come to use later (e.g. withrectangular, triangular, octagonal or eliptical crossection or withasymetric core) the splicer and connecting methods can be readilyadapted to such new fiber as long as its shape is precisely defined.

The method of connector manufacturing and optical fiber connectingaccording to the present invention has several essential steps:

1. The article of inherent shape memory polymer is created which hasbores which define one or more internal cavities of circular crossectionin positions corresponding to intended positions of the optical fibersto be connected (for instance, a single cylindrical cavity forend-to-end connection or splicing). The inherent inner cavity diameterIDI is the same or smaller than the outside diameter of the opticalfiber ODF (currently 125 microns for glass fibers). The cavity can becreated in a number of ways, such as drilling or burning the memorypolymer with laser beam and similarly. Particularly preferred method isto create the memory polymer around a Mandrel A of circular crossectionand outside diameter ODMA. Mandrel A can have different dimension thanthe optical fiber if some conditions are met (defined below), but it isimportant that the shape of crossection of the Mandrel A of the opticalfiber are the same and that the ratio ODMA/ODF has a certain presetvalue. This mandrel can be preferably the optical fiber itself, in whichcase ODMA=ODF and the conditions for Mandrel A are met automatically.The memory polymer unistructural mass can be created around such mandrelin a variety of ways. For instance, it can be created by polymeration ofsuitable mixture of monomers, including the crosslinking comonomer, in amold with the mandrel inserted. Alternatively, it can be created byembedding the mandrel into a non-crosslinked polymer precursor of thememory polymer, and by post-crosslinking the composition by some knownmethod depending on the chemistry of the memory polymer used (e.g.irradiation, heat, etc.).

Once the memory polymer is formed with the desired crosslinking density,the mandrel is removed. The removal can be carried out in various waysdepending on the mandrel properties. For instance, the mandrel can bepulled out provided that it has appropriate deformability (e.g., Nylon6), or can be melted, dissolved or etched away. The preferred process isto swell the memory polymer in a suitable solvent and pull the mandrelout from the enlarged cavity. After the polymerization and/orcrosslinking process is finished, memory polymer can contain, inaddition to the three-dimensional polymer network, an inert component,such as unreacted monomers or a diluent from the original mixture. Ifthe volume fraction of such diluent is v_(d), then the inherent innerdiameter of the cavity will be

    IDI=ODMA*(1-v.sub.d)1/3.                                   (2)

2. The cavity is enlarged and a new mandrel is inserted which has outerdiameter larger than ODF. This mandrel B can have crossection ratherdifferent from that of the optical fiber, but the optical fiber has tobe readily insertable into the cavity of the size and shape of theMandrel B. Any linear dimension of such cross-section has to be largerthan ODF. If the Mandrel B has circular cross-section, its diameter isdesignated ODMB. The ratio ODMB/ODF will determine ease of optical fiberconnection later. In any case, it is necessary that ODMB/ODF>1, andpreferably >1.1.

On the other hand, it is preferred that ODMB/ODF<2, because in oppositecase two optical fibers could fit accidentally into a cavity designedfor a single optical fiber. The preferred method of the cavityenlargement is the swelling of the inherent shape memory polymer. As arule, for any polymer there are a number of good solvents which willswell the memory polymer so that the Mandrel B is comfortably insertedinto the cavity. The necessary swelling can be readily calculated fromequations (1) and (2) or determined by experiment.

Mandrel B can be made of miscellaneous materials, such as polymers,metals or glass. From the viewpoint of the latter, for removal ofMandrel B, the partly extensible polymers are preferred (e.g.polyamides, polyurethanes or polyolefins).

3. The inherent shape memory polymer is shrunk around the Mandrel B byremoval of the solvent in which the memory polymer was swollen duringthe Mandrel B insertion. The solvent removal can be done by solventextraction or evaporation.

After the solvent removal is finished, the inner cavity diameter is ODMBand the residual volume fraction of solvent in the polymer is v_(s). Thev_(s) has to be such that

(a) T_(g) of the memory polymer is higher ambient temperature, andpreferably higher than 50° C.;

(b)

    v.sub.s <1-(1-v.sub.d)*(ODMA/ODF).sup.3                    (3)

The residual solvent can be left in the memory polymer intentionally forvarious reasons, e.g., to decrease T_(g) or other properties. If so, itis preferred to use in the previous step mixture of volatile andnon-volatile solvent in such a proportion that the latter remains inconcentration v_(s) after the volatile solvent is evaporated completely.

4. Mandrel B is removed from the cavity. This removal can be carried outin various ways, such as etching the mandrel away by an agent which doesnot attack the memory polymer; dissolve the mandrel in a solvent whichdoes not swell the memory polymer or does not decrease its T_(g) belowambient temperature, or similarly. The preferred way is pulling MandrelB made from a partly extensible polymer, out of the cavity.

After this operation the splicer is ready for installation of theoptical fiber. The Mandrel B removal can be carried out as a part ofmanufacturing process (i.e. prior to packaging the splicer) or justprior to the splice installation. The advantage of the latter method isincreased resistance of the splicer to an accidental temperatureincrease during storage or shipment, particularly if the inherent shapememory polymer mass has splicer to an accidental temperature increaseduring storage or shipment, particularly if the inherent shape memorypolymer mass has relatively low T_(s) for convenient installation. Thesplicing method is very simple due to the autoaligning characteristicsof the inherent shape memory polymer unistructural mass accordto theinvention.

The installation procedure consists of few essential steps:

1. Optical fibers are denuded in proper lengths (i.e., plastic buffer isremoved) and ends of fibers are prepared by the customary way (cleaving,polishing, etc.). This preparatory step is identical with otherconnecting and splicing methods.

2. Denuded and prepared ends of optical fibers are inserted into thecavities of the inherent shape memory polymer splicer. This step is veryeasy because the cavities have ID=ODMB which is larger than the receivedoptical fiber OD and no aligning or accurate positioning of the fibersis necessary.

3. While fibers are held in place, the splicer is heated above T_(s) fora time sufficient release of the internal stresses and consequentcontraction of the cavity to its final size and configuration, i.e. suchthat it goes from its deformed shape back to its inherent shape. Therate of such return can be very slow if the temperature is around glasstransition temperature T_(g) of the polymer. For this reason the polymerhas to be heated to a temperature sufficiently above T_(s) butsufficiently below the temperature at which the components could bedamaged. The cavity contracts around the fibers, as it tends to shrinkto the diameter

    IDC=ODMA*[(1-v.sub.d)/1-v.sub.s)].sup.166                  (4)

The cavity containing the optical fibers cannot shrink to a smallerdiameter than ODF, and holds the fibers by the force proportional to(ODF-IDC) difference.

The cavity contraction thus generates pressure forcing the fibers intothe intended position. The heating method is not particularly critical.The splicer may be heated by hot air, steam, electrically generatedheat, etc.

4. The connector is cooled down to a ambient temperature and secured bya suitable support or protective system, if necessary.

Referring now to the drawings there is shown in FIG. 1 Optical Splicer 1which is basically a mass of inherent memory polymer material which hasbeen formed with a bore 7 and a bore 9 at opposite ends 3 and 5 of itselongated dimension. As can be seen, bores 7 and 9 are, in fact, asingle bore of a single diameter in total alignment in a straight line.

FIGS. 2 through 6 show cut sideviews of an inherent shape memory polymermaterial through its various stages of preparation and use. Thus, inFIG. 2, there is shown unistructural mass 21a which has longitudinaldimension ends 23 with bores 25a and 29a as shown. In FIG. 2, bores 25aand 29a have been formed to create a single cylindrical tunnel or lumenthrough mass 21a and, as in FIG. 1, in fact, all form a single,continuous opening. The diameter of bores 25a and 29a are the same andare greater than the diameters of denuded fiber optic ends to be splicedas discussed in conjunction with FIG. 5 below. The bores 25a and 29a maybe formed by drilling, by other techniques which are discussed above, orby actually forming the unistructural inherent shape polymer materialmass around a mandrel of equal or slightly smaller size than the size ofthe optical fibers to be spliced. Next, the unistructural mass 21a isswollen with solvent and, as shown in FIG. 3, the expanded mass, that isunistructural mass 21b has a mandrel 45 inserted into it through bores25b and 29b which are of greater diameter than bores 25a 29a due to theswelling. Additionally, mandrel 45 is of greater diameter than thediameters of the end segments of the optical fibers to be spliced.Unistructural mass 21b is subsequently shrunk partially and mandrel 45is removed so as to create the unistructural mass 21c shown in FIG. 4.This unistructural mass 21c, with longitudinal ends 23 and 27 now havebores 25c and 29c. These bores are partially shrunk in diameter but arestill of greater diameter than the end segments of the optical fibers tobe spliced.

FIG. 5 shows unistructural mass 21c with ends 23 and 27 and bores 25cand 29c respectively. Here, optical fibers 33 and 37 have denuded endsegments 31 and 35. The end segments 31 and 35 are of less diameter thanbores 25c and 29c and are readily and freely inserted therein andcontacted in a butt-to-butt arrangement. Although not essential, variouspreparations for the tips or very ends of end segments 31 and 35 may beutilized prior to insertion and these techniques, such as cleaving arewell-known in the art. In any event, once optical fiber end segments 31and 35 have been inserted as shown in FIG. 5, unistructural mass 21c isreturned to its original inherent shape shown in FIG. 2 by virtue ofapplication of an external stimulus such as heat. This is shown in FIG.6, after heat has been applied, and, as can be seen, end segments 31 and35 are forced into excellent alignment and are held rigidly in placewith a permanent splice using the single unistructural mass without anymechanical parts, without any adaptors or screws or any other of thedevices which could create difficulties and/or minimize the useful lifeof the splice.

FIG. 7 shows a variation wherein unistructural mass 41 made of inherentshaped memory polymer material has a longitudinal dimension with ends 43and 45 opposite one another and bores 49 and 47 as shown. In this case,bore 49 has a smaller crossection than bore 47. Thus, bore 47 might bedesigned to accommodate a larger optical fiber or even one which has notbeen denuded.

EXAMPLE 1

All the percents in this and the following Examples are meant as percent by weight unless stated otherwise. Fifteen (15) cm long flint glasstube of I.D. 6 mm was closed by tightly fitting polyethylene caps oneach end. In the center of each cap, a small hole was punched to allowinsertion of optical fiber. Denuded and cleaned optical fiber (0.D. 125microns, core diameter 50 microns) was inserted through the hole of oneof the caps which was then fitted on the bottom end of the tube andsealed with an epoxy resin. Then two additional holes were punched intothe other cap to allow for filling by a syringe, and the cap wasinstalled on the top of the tube so that the optical fiber wasprotruding through its central hole. The mold thus formed was thenfilled by the monomer mixture consisting of 98.75% ofisopropylmethacrylate, 1.20% of tetraethyleneglycol methacrylate and0.05% of dibenzoyl peroxide. The filled mold was then fixed into a rackassuring that the optical fiber going through the mold is tight andstraight. The mold and rack were then put into a container filled withnitrogen, sealed and heated to 70° C. in oven for 14 hours. Afterwards,the mold was removed from rack and the glass tube was broken carefullyto extract the rod-shaped piece of the clear, rigid polymer with opticalfiber embedded approximately in the rod axis. The polymer rod was thenswelled in methyl alcohol overnight so that it expanded somewhat andbecame rubbery. The glass fiber was readily removed at this point. Therod was cut by a razor to 10 mm sections which were then swelledgradually in methylisobutylketon. After the complete swelling theinternal cavity diameter was approximately 200 microns so that readilyaccepted insertion of Nylon 6 fiber of diameter about 150 microns. Withthe Nylon mandrel in place, the splicers were deswelled in methanol andthen dried first at 105° C. for about 12 hours and cooled to ambienttemperature. After this time the polymer became again clear and rigid,and the Nylon fiber was readily extracted by moderate pull. Theresulting splicer had a cavity of about 150 microns approximately in thecenter. The polymer had T_(g) =81.3° C. as determined bythermomechanical measurement. The splicers were used splicing theoptical fiber of the same specifications as the one used as mandrel A.Fiber ends were prepared by denuding in the customary was and cleavedwith a commercial cleaver. Then the fiber ends were inserted into thesplicer bore until they met, and while held in place, the splicer washeated with a commercial heat-gun for several seconds and then left tocool to ambient temperature (total time of heating and cooling was about1 minute). Then the insertion loss of the spliced fibers was measured onan optical bench using 5 mW HeNe laser as a source and silicone planardiffused PIN photodiode as a detector. The loss was calculated fromcomparison between the spliced fiber and the original continuous fiberof the same length. It was found that the spliced fibers had aconsistently lower insertion loss than 0.5 deciBell (dB).

EXAMPLE 2

The same molds as in example were filled with a monomer mixtureconsisting of: 76.85% of isopropylmethacrylate, 20% ofn-Butylmethacrylate, 3.1% of ethyleneglycol dimethacrylate and 0.05% ofdibenzolylperoxide. The mixture was polymerized under the conditionsdescribed in Example 1. After the swelling the polymer in methanol andextracting the optical fiber, the swollen rod was cut by razor inparallel cuts under angle about 45° and dried in oven until rigid. Thenthe angled faces were ground and polished to be uniformly smooth, andthe splicers were reswollen in methanol. The rest of the process wasidentical with Example 1. The splicers had T_(g) about 70° C. and theirangled, polished faces facilitated insertion of the optical fibersconsiderably, even though some splicers had off-centered bores. Thespliced fibers (using dry fibers without any coupling agents) had thefollowing performance:

Average insertion loss: 0.28 dB

Standard deviation: 0.14 dB

Average experimental error: 0.03 dB.

The splicing procedure including fiber preparation took less than 5minutes. This example shows that the splicers from the unistructuralmass according to our invention can in themselves provide low losssplicing in spite of assymetric shape and without using variousauxilliary means (coupling gels, index-matching adhesives, mechanicalsupports, etc.) the other splicers need for a comparable performance.

EXAMPLE 3

Aluminum mold was prepared in the following way: two rectangular holes(about 90×20 mm) were cut into a rectangular aluminum plate 100×50 mmlarge and about 5 mm thick so that about 2 mm wide strip was leftbetween both apertures. Short lengths (about 50 mm) of denuded opticalfiber (0.D. 125 microns) were then laid across the width of the plateand glued to it by epoxide glue on each end. Since the fibers wererelatively short and supported in the middle by the 2 mm aluminum strip,no external mechanism was needed to straighten them or stretch them.This part was then put into a flat-bottomed aluminum weighting boat andfilled with the same monomer mixture as in Example 2. The volume of themonomer mixture was selected so that the fibers were approximately inthe middle of the layer. Then the mold was flushed with nitrogen andcovered by an aluminum weighting boat of the same size which floatedfreely on the monomer. The mold was put into nitrogen-filled containerand polymerized as in previous examples. After the polymerization wasfinished, both boats were stripped from the frame plus polymer, and thepolymer was let to swell in methanol. The swollen polymer was then takenfrom the frame, the sections of the fiber were pulled out and the stripsof polymer were cut in parallel fashion so that the bores left behind bythe optical fibers were roughly in the middle between and parallel tothe cuts. The sections were then cut again by razor to a shapeapproximating a trapezoid, and dried. The dry pieces were then groundand polished into the shape of a trapezoid having a larger rectangularface about 10×4 mm opposite smaller rectangular face about 2×2 mm heightabout 4 mm. The bore axis was about 2 mm above the largest face andlength about 6 mm. The splicer precursors were reswollen in methanol,and then swelled consecutively in ethanol, isopropanol and acetone (inthe order of increasing swelling). Once the maximum swelling in acetonewas achieved, the bore had a diameter over 200 microns and acceptedreadily Nylon mandrel of diameter about 175 microns. The polymer overthe mandrel was then carefully deswelled using above swelling solventsin opposite order, and dried as in the previous examples. The clear,homogeneous splicers of the trapezoidal shape were stored with the Nylonmandrel inside to protect the bore from contamination. The mandrel wasreadily pulled out immediately prior splicing. The splicers were moreconvenient to use than the rod-shaped splicers described in the previousExamples; and in spite of utter lack of symmetry, they had a similar lowinsertion loss without using coupling agents or auxilliary supportstructures. The above examples specificly illustrate some of thepreferred embodiments of the splicer of the present invention, includingits preparation and its use. Obviously, variations may be made by theartisan with out exceeding the scope of the present invention. Thefollowing examples illustrate testing and polymer variations, asfollows:

EXAMPLES 4 AND 5

The procedure to analyze the performance of the splicer was to measurethe optical loss suffered by a continuous fiber after the fiber has beencut in half and then reconnected by the splicer. The experimental set-upconsisted of a source, mechanical apparatus to hold the fiber and alignit with the source, and a power meter to measure the amount of lightpropagated through the fiber.

The source used was a 5 mW HeNe laser chosen for its ease of beammanipulation and excellent amplitude stability. The output of the HeNelaser was first steered through a light baffle to insure that diffusescattering of the beam from some of the optical components used(primarily the input coupler) would not be reinjected into the lasercavity. Retroreflected components of the laser beam can cause rapidfluctuations in the laser intensity due to power drop outs caused bymodal competition between the internal cavity modes and modes externalto the cavity (scattered components). A 0.5 neutral density filter (32%transmission) was used as the primary element in the baffle. The outputof the baffle was steered into the fiber optic coupler. The couplerfocused the beam into the input end of the fiber which was held by thecoupler. The fiber now served as the sole propagation link of thesystem. The output end of the fiber was held by the output collimatorwhich served to mechanically hold the fiber and collimate the lightemerging from it to illuminate an optical detector. The optical detectorconsisted of a silicon planar diffused PIN photodiode whose output wasfed into a digital power meter for visual display.

The procedure for measuring the performance with this equipment was asfollows:

1. Approximately one meter length of optical fiber was prepared byremoving (buffer swollen in dichloromethane) about 150 mm of buffer fromeach end of the fiber and the ends cleaved to produce clean,perpendicular faces. The exposed length of each end (buffer removed) wason the order of 100 mm.

2. The fiber ends were then cleaned by wiping with isopropyl alcohol andthen inserted into chucks, which held the fiber. One chuck was mountedto the input coupler and the other to the output collimator.

3. The input coupler was then adjusted to achieve the maximum powerthrough the fiber (1.5-1.8 mW). This power was then recorded (I_(o)).

4. The fiber was then cut in the middle and the cut ends prepared as in(1) except only 50 mm of buffer was initially removed and the exposedlengths were less than 10 mm.

5. The cut ends were then inserted into a splicer which was heated toeffect the memory action.

6. After the splice was heated by a commercial heat gun and allowed tocool (total time approximately 60 seconds), the resultant power throughthe fiber was the recorded (I).

7. Loss was calculated in dB: ##EQU1## Insertion losses measured oncandidate splices averaged 0.28 dB as discussed in conjunction withExample 2 above. It is believed that the splice performance can befurther improved to the point of an average loss of 0.2 dB with a muchnarrower standard deviation. The performance achieved compares quitefavorably with the performance of other multimode splices. Thus, theperformance work of 0.2 dB insertion loss may be achieved with thepresent invention in the field whereas this is usually not achieved withthe prior art techniques. The only mechanical splice reported to havesimilar performance without a coupling cement or gel is the GTEelastomeric splice which is very expensive due to the amount ofprecision molding necessary to produce the splice. This splice makes useof a compliant material in which a precision cavity is molded ormachined. The material deforms to allow insertion of the fibers and thenrelies on elastic forces to align and fix the fibers. Performance datafrom Example 2 is shown in Table 1:

                  TABLE 1                                                         ______________________________________                                        PERFORMANCE DATA                                                              ______________________________________                                        SAMPLE SIZE:            N = 7                                                 AVERAGE INSERTION LOSS  Less than 1.0 dB                                      TARGET:                                                                       *MEASURED AVERAGE INSER-                                                                              0.28 dB                                               TION LOSS:                                                                    AVERAGE EXPERIMENTAL ERROR:                                                                           0.03 dB                                               STANDARD DEVIATION:     0.14 dB                                               NOMINAL RECOVERY TIME                                                         OF SPLICE                                                                     TO ALIGN AND SECURE     30 seconds                                            CLEAVED FIBERS:                                                               AVERAGE T.sub.g (glass  81.3° C.                                       transition temperature):                                                      ______________________________________                                         *All losses measured on optical fiber with 50 micron core diameter and        total cladding diameter of 125 micron. Multimode silica glass fibers were     tested using a 633 nm source.                                            

EXAMPLES 6-12 POLYMER COMPOSITION MIX AND SOLVENTS

Table 2 shows various glass transition temperatures calculated (T_(g))for formulations 6-0 through 12-0 and Table 3 illustrates the activityof various solvents on crosslinked IPMA with 1% TEGDMA. These merelyillustrate the types of variations which may occur regarding T_(g) basedon changing polymer component percentages and based on solventvariations. Table 4 shows swelling changes for solvent and component mixchanges simultaneously.

The time taken to make a splice, as outlined in a previous section, isdominated by the time it takes to sufficiently swell the buffer indichloromethane in order to remove it from the fiber. In actual fieldapplications, this procedure can be easily shortened by usingcommercially available mechanical strippers

                                      TABLE 2                                     __________________________________________________________________________    COMPOSITION OF MEMORY POLYMER FOR FIBER OPTIC SPLICE                                               Tetraethylene-                                                  Isopropyl                                                                            n-Butyl                                                                              glycolmetha-                                             Formulation                                                                          Methacrylate                                                                         Methacrylate                                                                         crylate Tg                                               No.    Wt. %  Wt. %  Wt. %   Calculated                                                                          Measured                                   __________________________________________________________________________    6 - 0  98.8   0      1.20    81.2  81.3                                       7 - 0  88.6   10.2   1.20    75.1  --                                         8 - 0  87.9   10.0   2.10    75.4  --                                         9 - 0  78.7   20.3   1.0     69.0  --                                         10 - 0 97.0   0      3.0     81.6  --                                         11 - 0 86.9   10.0   3.1     75.6  --                                         12 - 0 76.9   20.0   3.1     69.6  --                                         __________________________________________________________________________

                  TABLE 3                                                         ______________________________________                                        SWELLING OF CROSSLINKED ISPROPYL METHACRY-                                    LATE (1% OF TEGDMA) IN SELECTED SOLVENTS                                      SOLVENT     L/L.sub.o  Bs, % Wt.  REMARK                                      ______________________________________                                        Methanol    1.21       39.3       Flexible                                                (Calc. 1.24)                                                      Ethanol     1.33       61.2       Flexible                                    Isopropanol 1.42       70.7       Flexible                                    Acetone     1.63       75.2       Cracks                                      MeIBK       1.62       77.5       Flexible                                                (Calc. 1.60)                                                      Cyclohexane 1.67       79.1       Some                                                                          Cracks                                      Morpholine  1.45       69.2       Cracks                                      Tetrachloroethylene                                                                       1.84       89.5       Very                                                                          Brittle                                     2-Me Pyrrolidone                                                                          1.53       76.5       Cracks                                      Butyrolaceton                                                                             1.25       55.6       Soft                                        DMSO        1.03       18.8       Rigid                                       ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        LINEAR SWELLING OF MEMORY POLYMER COMPO-                                      SITIONS IN VARIOUS SOLVENTS (L/L.sub.o) COMPILED                              FROM TABLE 1:                                                                 Solvent 6-0     7-0    8-0   9-0  10-0  11-0 12-0                             ______________________________________                                        MeOH    1.21    1.25   1.21  1.27 1.19  1.20 1.22                             EtOH    1.33    --     --    1.40 1.33  --   1.37                             IPOH    1.42    1.52   1.50  1.60 1.43  --   1.57                             Acetone 1.63    1.58   1.57  1.60 1.48  1.55 1.55                             MeIBK   1.62    1.64   1.63  1.66 1.53  1.59 1.61                             ______________________________________                                    

Obviously, numerous modifications and variations of the presentinvention are possible in light of the above teachings. It is thereforeunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specifically described herein.

What is claimed is:
 1. An optical fiber splicer which comprises: aunistructural mass of inherent shape memory polymer material having alongitudinal dimension with opposite ends which includes a first bore atone end of said opposite ends into the inside of said mass, and a secondbore at the other end of said opposite ends into the inside of said massto and in alignment with said first bore, said mass having a first shapeand a second shape:(a) said first shape being a unique, recoverable,predetermined inherent shape wherein said first bore and said secondbore each have a preset diameter to accommodate and tightly hold endsegments of denuded optical fibers of predetermined diameter in spliced,butted alignment with one another; and, (b) said second shape being adeformed shape caused by solvent swelling of said inherent memorypolymer material wherein said first bore and said second bore each havea swollen predetermined diameter greater than the diameter of theaforesaid end segments of denuded optical fibers so as to loosely andfreely receive said end segments of said denuded optical fibers;saidmass of inherent shape memory polymer material having been initiallyformed in said first shape (inherent shape) and having been swollen thenpartially shrunk to said second shape (deformed shape), and capable ofbeing returned to said first shape by application of a non-mechanicalstimulus thereto.
 2. The optical fiber splicer of claim 1 wherein saidfirst bore and said second bore are of different size diameter in theirinherent shape and said bores have central axes which form a singlestraight line.
 3. The optical fiber splicer of claim 1 wherein saidfirst bore and said second bore are of the same size diameter in theirinherent shape and have central axes which form a single straight line.4. The optical fiber splicer of claim 1 wherein the said unistructuralmass is crosslinked amorphous organic polymer with a glass transitiontemperature higher than 25° C. but lower than 200° C.
 5. The opticalfiber splicer of claim 4 wherein the said amorphous crosslinked polymerhas a glass transition temperature between 50° C. and 150° C.
 6. Theoptical fiber splicer of claim 4 wherein the said amorphous crosslinkedpolymer consits of at least one monomer selected from derivatives ofacrylic acid, methacrylic acid and styrene.
 7. The optical fiber splicerof claim 6 wherein the said derivative is ester of methacrylic acid andalcohol with one to ten carbon atoms.
 8. The optical fiber splicer ofclaim 1 wherein said unistructural mass has a shape which isasymmetrical with respect to the bore axis.
 9. The optical fiber splicerof claim 8 wherein no surface of the unistructural mass is perpendicularto the bore axis.
 10. The optical fiber splicer of claim 9 wherein saidunistructural mass is trapezoidal and the largest of its rectangularfaces is about parallel to the bore axis.
 11. A method of preparing anoptical fiber splicer which comprises:(a) polymerizing a monomer mixtureof inherent shape memory polymer so as to create a unistructural masshaving a longitudinal dimension with opposite ends which includes afirst bore at one end of said opposite ends into the inside of saidmass, and a second bore at the other end of said opposite ends into theinside of said mass to and in alignment with said first bore, each ofsaid bores having a predetermined cross-section equal to or less thanthe cross-section of selected denuded optical fibers; (b) swelling theinherent shape memory polymer to an enlarged size such that each of saidbores has an enlarged cross-section which is greater than thecross-section of said selected denuded optical fibers; (c) partiallyshrinking the swollen inherent shape memory polymer by solvent removalto a deformed shape such that each of said bores has a predeterminedcross-section which is less than the fully swollen cross-section butnonetheless greater than the cross-section of said selected denudedoptical fibers.
 12. The method of claim 11 wherein said polymerizing iscarried out to create said unistructural mass by being reacted about oneor more selected, denuded optical fibers.
 13. The method of claim 11wherein said swelling is performed by soaking in one or morethermodynamically good solvents.
 14. The method of claim 12 wherein saidswelling is performed by soaking in one or more thermodynamically goodsolvents.
 15. The method of claim 11 wherein, after said swelling, atleast one mandrel is inserted into said bores which has a cross-sectiongreater than the cross-section of said selected, denuded optical fiberand then said shrinking is performed while said mandrel is inserted, andfurther wherein said mandrel is removed after said shrinking.
 16. Themethod of claim 12 wherein, after said swelling, at least one mandrel isinserted into said bores which has a cross-section greater than thecross-section of said selected, denuded optical fiber and then saidshrinking is performed while said mandrel is inserted, and furtherwherein said mandrel is removed after said shrinking.
 17. The method ofclaim 13 wherein, after said swelling, at least one mandrel is insertedinto said bores which has a cross-section greater than the cross-sectionof said selected, denuded optical fiber and then said shrinking isperformed while said mandrel is inserted, and further wherein saidmandrel is removed after said shrinking.
 18. The method of claim 11wherein said monomer mixture is selected from derivatives of acrylicacid and methacrylic acids.
 19. The method of claim 18 wherein saidderivatives are esters of alcohols.
 20. The method of claim of 19wherein said alcohols have 1 to 10 carbon atoms.
 21. The method of claim20 wherein said monomer mixture includes methylmethacrylate andbutylmethacrylate.
 22. The product resulting from the method of claim11.
 23. The product resulting from the method of claim
 12. 24. Theproduct resulting from the method of claim
 13. 25. The product resultingfrom the method of claim
 14. 26. The product resulting from the methodof claim
 15. 27. The product resulting from the method of claim
 16. 28.The product resulting from the method of claim
 17. 29. The productresulting from the method of claim
 18. 30. The product resulting fromthe method of claim
 19. 31. The product resulting from the method ofclaim
 20. 32. The product resulting from the method of claim
 21. 33. Amethod of splicing optical fibers in a butt-to-butt fashion, whichcomprises:(a) inserting end segments of denuded optical fibers to bespliced into an optical fiber splicer having a unistructural mass ofinherent shape memory polymer material having a longtudinal dimensionwith opposite ends which includes a first bore at one end of saidopposite ends into the inside of said mass, and a second bore at theother end of said opposite ends into the inside of said mass to and inalignment with said first bore, said mass having a first shape which isunique, recoverable, predetermined inherent shape wherein said firstbore and said second bore each have a preset diameter to accommodate andtightly hold end segments of denuded optical fibers of predetermineddiameter to accommodate and tightly hold end segments of denuded opticalfibers of predetermined diameter in spliced, butted alignment with oneanother; and, said mass having a second shape which deformed shapecaused by solvent swelling of said inherent memory polymer materialwherein said first bore and said second bore each have a swollenpredetermined diameter greater than the diameter of the aforesaid endsegments of denuded optical fibers so as to loosely and freely receivesaid end segments of said denuded optical fibers;said mass of inherentshape memory polymer material having been initially formed in said firstshape (inherent shape) and having been swollen then partially shrunk tosaid second shape (deformed shape) prior to the insertion of said end tosegments of said optical fibers; and, (b) after said insertion, applyinga non-mechanical stimulus to said unistructural mass so as to return toit its inherent shape to align and hold the spliced end segments of saidoptical fibers.
 34. The method of claim 33 wherein said optical fibersplicer bores are of the same diameter and have central axes which forma single straight line.
 35. The method of claim 33 wherein saidnon-mechanical stimulus is heat.
 36. The method of claim 34 wherein saidnon-mechanical stimulus is heat.